1. Field of the Invention
The present invention relates to an image forming apparatus capable of correcting unevenness of an image in a main scanning direction.
2. Description of the Related Art
An image forming apparatus may provide various image qualities such as graininess, uniformity in a plane, character quality, and reproducibility (including color stability). Such image qualities provided by an electrophotography image forming apparatus may be influenced by uneven electrification caused by degradation of a charger which electrostatically charges a photosensitive drum, uneven exposure of a laser scanner, for example, configured to form an electrostatic latent image on a photosensitive drum, uneven development by a developing device which develops an electrostatic latent image or the like.
These unevennesses may cause uneven density and/or uneven color in a main scanning direction (orthogonal to a sheet conveying direction for forming an image on a sheet), which may disadvantageously deteriorate uniformity in a plane.
Japanese Patent Laid-Open No. 2004-163216 proposes a technology (main-scanning shading correction) of outputting a sheet on which a plurality of test patterns are printed in a main scanning direction and measuring color densities of the test patterns with a handy densitometer, for example, to correct an uneven density in the main scanning direction.
On the other hand, Japanese Patent Laid-Open No. 2006-58565 discloses a method of performing such main-scanning shading correction by using a color sensor internally mounted in an image forming apparatus.
Japanese Patent Laid-Open No. 2006-58565 discloses a technology of forming a band-shaped test pattern based on an equal image signal value in a main scanning direction of a sheet. Japanese Patent Laid-Open No. 2006-58565 further discloses a technology of rotating a sheet having a test pattern 90 degrees, setting it to a feeder, refeeding the sheet, and measuring the test pattern by using a color sensor within an image forming apparatus.
However, a user may be required to determine whether a sheet having a test pattern is to be set with its face up or down and/or by rotating 90 degrees to the right or to the left in accordance with the feeder in which the sheet is to be set.
This may require a user to set a sheet in consideration of the side and right or left direction of the sheet, which lowers user's operability. When such a sheet is set in a wrong direction in a feeder, a correct measurement result may not be acquired from the test pattern. In such a case, the user must set the sheet in the feeder again, which may cause user stress.